FIG. 13 illustrates a cross-sectional view of a junction field-effect transistor (JFET) disclosed in U.S. Pat. No. 7,560,325. The JFET is made of silicon carbide (SiC), which is a WBG semiconductor material suitable for high frequency and high temperature applications. As shown in FIG. 13, a P−-type buffer layer J2 is formed on a silicon carbide substrate J1, a N−-type channel layer J3 is formed on the P−-type buffer layer J2, and a N+-type layer J4 is formed on the N−-type channel layer J3. A recess J5 extending from a surface of the N+-type layer J4 to the N−-type channel layer J3 is formed by etching the N+-type layer J4. A P+-type gate region J7 is formed in the recess J5 through a P−-type layer J6. A source electrode J9 and a drain electrode J10 are formed through a metal layer J8 in such a manner that the source electrode J9 and the drain electrode J10 are spaced from the P+-type gate region J7.
In U.S. Pat. No. 7,560,325, the recess J5 is formed by etching the N+-type layer J4 without using a stopper layer. Therefore, the etching of the N+-type layer J4 is stopped simply by controlling an etching time. If the N+-type layer J4 is partially left, and the recess J5 does not reach the N−-type channel layer J3, switching characteristics can significantly vary. Therefore, the etching time is determined so that the recess J5 can surely reach the N−-type channel layer J3. In this case, there is a fear that the N−-type channel layer J3 will be over etched. As a result, the thickness of the N−-type channel layer J3 may vary, and characteristics of the JFET may vary.
The above discussion regarding the thickness variation of a channel layer can be applied to a metal-semiconductor field-effect transistor (MESFET) and a metal-oxide-semiconductor field-effect transistor (MOSFET).
In view of the above, it is an object of the present invention to provide a WBG semiconductor device having a JFET, a MESFET, or MOSFET in which a variation in thickness of a channel layer can be reduced. It is another object of the present invention to provide a method of manufacturing the WBG semiconductor device.